A gas-insulated electrical device includes a cylindrical ground tank and a current-carrying cylindrical high-voltage conductor disposed coaxially in the ground tank, and the ground tank is hermetically filled with an insulation gas represented by a chief insulation medium, such as a sulfur hexafluoride gas, dry air, nitrogen, a carbon dioxide gas, CF4, CHI3, C2F6, and C3F8, or a mixed gas obtained by combining the foregoing gases. The gas-insulated electrical device configured as above is chiefly used as a high-voltage device. In particular, a sulfur hexafluoride gas has a dielectric strength about three times higher than that of air. Accordingly, when a sulfur hexafluoride gas is employed, a distance between a high voltage portion and a ground electrode can be shorter and hence a size of the device can be reduced.
Normally, a gas used as an insulation medium in the gas-insulated device is pressurized to or above an atmospheric pressure in order to enhance insulation performance and breaking performance. Therefore, in order to hermetically seal the gas and maintain an equal insulation distance, the gas-insulated device adopts a structure in which a tank forming the airtight container described above is of a cylindrical shape and a high-voltage conductor of also a cylindrical shape is disposed coaxially with the tank.
While a sulfur hexafluoride gas has a very high dielectric strength, it should be noted that insulation performance deteriorates under an inhomogeneous electric field. In the case of switchgear, metal foreign matter in the order of millimeters may possibly be generated from a sliding portion where metal is joined to metal and a contact portion where conductors, such as a breaker and a disconnector, are in contact with each other. The metal foreign matter thus generated first rests on a bottom surface in the tank forming the airtight container. The metal foreign matter, however, becomes electrically charged due to an action, such as electrostatic induction, and starts to move according to a potential gradient between the tank and the high-voltage conductor while the device is in operation. In the beginning, the metal foreign matter that has been lying flat stands up. In the case of a DC device, in particular, the metal foreign matter standing up starts to float and moves so as to eventually come in close proximity to the high-voltage conductor or make contact with the high-voltage conductor. The metal foreign matter accumulates while behaving in the vicinity of the conductor particularly when the high-voltage conductor is charged to a negative polarity. In the coaxial cylindrical shape, the electric field becomes highest in the vicinity of the high-voltage conductor. Hence, the metal foreign matter becomes extremely inhomogeneous and the electric field becomes high in the vicinity of the metal foreign matter. When an overvoltage, such as a lightning surge, flows into the device in such a state, a ground fault may possibly occur.
An effective countermeasure against a decrease of a withstand voltage in the presence of the metal foreign matter is a tank inner surface structure, by which incoming metal foreign matter is forced to rest on the tank bottom surface as much as possible to suppress the foreign matter from floating. More specifically, this is a method to enhance withstand voltage performance against foreign matter by providing an insulation material of at least 0.2 mm thick to the inner surface of a tank wall (see, for example, PTL 1).
Also, an amount of charge of the foreign matter is suppressed by providing a non-linear resistance film on an inner surface of the tank to suppress the foreign matter from floating (see, for example, PTL 2).